Reference Manual

1−4
Figure 1-6. High Capacity Valve Body with
Cage-Style Noise Abatement Trim
W0997
Downstream pressure acts upon both the top and
bottom sides of the valve plug, thereby nullifying
most of the static unbalance force. Reduced
unbalance permits operation of the valve with
smaller actuators than those necessary for
conventional single-ported valve bodies.
Interchangeability of trim permits the choice of
several flow characteristics or of noise attenuation
or anticavitation components. For most available
trim designs, the standard direction of flow is in
through the cage openings and down through the
seat ring. These are available in various material
combinations, sizes through NPS 20, and pressure
ratings to Class 2500.
High Capacity, Cage-Guided Valve
Bodies
This adaptation of the cage-guided bodies
mentioned above was designed for noise
applications, such as high pressure power plants,
where sonic steam velocities are often
encountered at the outlet of conventional valve
bodies (figure 1-6).
The design incorporates oversized end
connections with a streamlined flow path and the
ease of trim maintenance inherent with cage-style
constructions. Use of noise abatement trim
reduces overall noise levels by as much as 35
decibels. The design is also available in cageless
versions with a bolted seat ring, end connection
sizes through NPS 20, Class 600, and versions for
liquid service. The flow direction depends upon the
intended service and trim selection, with
unbalanced constructions normally flow-up and
balanced constructions normally flow-down.
Port-Guided Single-Port Valve Bodies
D Usually limited to 150 psi (10 bar) maximum
pressure drop.
D Susceptible to velocity-induced vibration.
D Typically provided with screwed in seat rings
which might be difficult to remove after use.
Three-Way Valve Bodies
D Provide general converging (flow-mixing) or
diverging (flow-splitting) service.
D Best designs use cage-style trim for positive
valve plug guiding and ease of maintenance.
D Variations include trim materials selected for
high temperature service. Standard end
connections (flanged, screwed, butt weld, etc.) can
be specified to mate with most any piping scheme.
D Actuator selection demands careful
consideration, particularly for constructions with
unbalanced valve plug.
A balanced valve plug style three-way valve body
is shown with the cylindrical valve plug in the down
position (figure 1-7). This position opens the
bottom common port to the right-hand port and
shuts off the left-hand port. The construction can
be used for throttling mid-travel position control of
either converging or diverging fluids.
Rotary Valves
Traditional Butterfly Valve
Standard butterfly valves are available in sizes
through NPS 72 for miscellaneous control valve
applications. Smaller sizes can use versions of
traditional diaphragm or piston pneumatic
actuators, including the modern rotary actuator
styles. Larger sizes might require high output
electric or long-stroke pneumatic cylinder
actuators.
Butterfly valves exhibit an approximately equal
percentage flow characteristic. They can be used